The present invention is directed to a reframing method and apparatus therefor for a carrier system having a serial digital bit stream and more particularly to a telephone type pulse code carrier system of the D2 or D3 type used in T1 systems.
In the D2/D3 PCM line format each frame consists of 193 bits; the first bit being a framing bit (BF or DF) the remaining 192 bits consisting of 24 channels of data with eight bits per channel. As is well known in the art each eight bit channel of a particular frame represents the analog voltage level of an incoming voice signal. Twelve such frames form a multiframe.
Within each multiframe the framing bit is divided into two codes; first the terminal frame code, F.sub.T, which is presently used for frame detection and reframing and which consists of an alternating patter of one and zero bits every other frame; and secondly, the framing bits form a signalling frame code, F.sub.S, which is used for signaling frame detection and consists of three 0 bits followed by three 1 bits, again every other frame. The two frame bit codes taken together repeat themselves every 12 frames which forms a multi-frame. FIG. 1 illustrates the foregoing patterns.
Since only the F.sub.T or terminal frame code is utilized for reframing and detecting framing errors single frequencies such as a 2 kHz tone which are transmitted over the carrier system may cause false reframing. In addition, merely using the terminal frame code for reframing incurs excessive time delays.